Implemented "from/to imperial units conversation" for loaded objects and volumes

src/slic3r/GUI/Plater.hpp

@@ -212,6 +212,7 @@ public:
     void set_number_of_copies(/*size_t num*/);
     bool is_selection_empty() const;
     void scale_selection_to_fit_print_volume();
+    void convert_unit(bool from_imperial_unit);
 
     void cut(size_t obj_idx, size_t instance_idx, coordf_t z, bool keep_upper = true, bool keep_lower = true, bool rotate_lower = false);